Models of cation exchange mechanisms and driving forces have proven effective predictors of clay behavior and chemistry, but are largely theoretical, particularly in complex systems involving high ionic strength brines or systems where hydration is controlled by relative humidity. In arid and cold environments, such as Mars, cyclical relative humidity variations may play a role in chemical alteration, particularly if clay minerals such as smectite are in the presence of salts. This study examines the effects of relative humidity on smectite-salt mixtures using environmental scanning electron microscopy (ESEM) to observe the physiochemical effects of salt deliquescence and desiccation on smectite textures and elemental distributions. Results demonstrate that even reaction periods as short as a few minutes allow ample time for relative humidity to affect the smectite-salt mixtures. In addition to smectite swelling and salt deliquescence, we also observed rapid changes in element distributions within the smectite and new crystal growth in the presence of high relative humidity. Even in the absence of bulk liquid water, exchangeable cations migrated out of the smectite and formed new crystals at the smectite-salt interface. The observed microscopic changes in elemental distributions indicate that the migration of cations driven by cation exchange led to secondary mineral precipitation, likely a CaSO4 mineral, within a sub-micrometer-thick layer of water on the smectite grains. The results of this study demonstrate that during periods of elevated relative humidity, active smectite mineral alteration and secondary mineral precipitation may be possible on present-day Mars where salts and smectites are in direct physical contact.

The structure and hydration status of attapulgite clay after heating at elevated temperatures and the stability of parathion on these clays was studied. Using infrared spectroscopy and scanning electron microscopy it was found that the bound water was lost in two steps, at 250° and 450° with the first step being largely reversible. At 650°C the structure began to dissolve releasing significant amounts of Mg, and a decrease in aggregate porosity was noted. At 850°C an amorphous phase was formed bearing little resemblance to the original attapulgite. Parathion was stable on all of the preheated clays when kept at 25°C for 190 days. The reactions of parathion on the preheated clays was studied at 110°C Hydrolysis of parathion was found to be minimal. Isomerization was the main reaction occurring on the Ca-attapulgite, whereas on an organo-clay no isomerization was observed. A mechanism for the isomerization reaction is proposed which entails a distortion of the phosphate moiety of the pesticide by the oxygen of the ligand water resulting in the conformational changes necessary for the isomerization to take place. On the organo-clay such a conformation was not possible; hence no isomerization occurred.

Laterite profiles developed from granite in southwestern Australia were studied by scanning electron microscopy. The morphology of soil materials reflects the mineralogy of secondary minerals formed from feldspar. In the saprolite, etched feldspar surfaces are coated with kaolinite or radiating, spherical aggregates of tubular halloysite. In the lower pallid zone these minerals have replaced most of the feldspar. In the upper pallid zone a porous framework has developed consisting mainly of quartz and gibbsite with 5-/μm euhedral gibbsite crystals in voids. Halloysite crystals in the upper pallid zone are partly unrolled and have splayed ends. Differences in mineralogy and morphology between profiles are thought to be due to variations in the intensity of leaching.

The conversion of smectite to illite by way of a mixed-layer illite/smectite (I/S) series was found to be the major depth-related reaction in clay-mineral assemblages from two cored sedimentary sequences in the Rocky Mountains. The I/S reaction occurred in both interbedded sandstone and shale of Upper Cretaceous and lower Tertiary age in the Green River basin, Wyoming, and in chalk and chalky shale of the Upper Cretaceous Niobrara Formation, Denver basin, Colorado. As the proportion of illite layers in I/S increased with depth in these rocks, the amount of I/S in the clay fraction decreased, and the amount of discrete illite increased. Scanning electron microscopy revealed that the morphologies of highly expansible, randomly interstratified I/S clay (samples from shallow cores) exhibited no distinctive intergrowth or overgrowth textures. In deeply buried rocks containing highly illitic, ordered I/S and abundant discrete illite, however, fibers or laths of illite were formed on earlier I/S substrates. Less commonly, I/S of low expandability shows morphological features of both smectite and illite whereby rigid laths of illite appear to have formed diagenetically from the wall surfaces of I/S honeycombs. This combined morphology suggests some dissolution and reprecipitation (or some reorganization) of materials from the I/S substrate as the substrate was transformed into a more illitic mixed-layer clay.

These data suggest that some I/S clay was destroyed by the selective cannibalization of smectite layers in I/S to provide the components needed to make a more illitic I/S. Moreover, discrete illite and other minerals apparently were formed during the reaction. Also, coarser mineral phases, such as potassium feldspar and detrital micas, may not have been required to supply the chemical components in the reaction. The observations provide an explanation for late diagenetic I/S reactions that occurred in restricted or relatively closed geochemical systems, as in early cemented rocks having extremely low permeability and little or no potassium feldspar.

During an electron microscope study of the weathering of granite from southern New South Wales, Australia, an assemblage of minerals including florencite and rhabdophane was discovered replacing apatite. Light rare earth elements released from allanite early in weathering apparently combined with P released by leaching of primary apatite to form secondary phases exhibiting a range of morphologies and compositions. Chondrite normalized fractionation patterns La > Nd > Sm > Ce; La > Ce > Nd ≥ Sm and La = Sm = Nd > Ce were identified. The rare earth elements were present in very small crystals and aggregates of secondary minerals (< 10 μm) and coexisted with clays and secondary Fe-Ti oxides. The weathered granite was enriched about 6 to 10 times in all rare earth elements except Ce relative to fresh granite if the abundances were corrected for apparent enrichment due to reduction in density. The rare earth elements were probably derived from higher in the weathering profile, possibly by destruction of florencite and rhabdophane in very intensively weathered rock. Ce remained relatively immobile during weathering, probably due to its oxidation to Ce4+.

Sedimentologic zones that are differentiated by changes in lithology, mineralogy, chemical composition, and crystal morphology observable in scanning electron micrographs occur in Missouri high-alumina clay deposits. These properties and changes suggest that the high-alumina materials originated from Pennsylvanian-age, paludal sediments deposited in depressions underlying Paleozoic carbonate rocks. Alumina was relatively enriched in zones of the deposits by leaching of silica and of alkali and alkaline earth metals from the sediments. The most intense leaching occurred on the highest parts of the Ozark Dome.

Diaspore is the predominant high-alumina mineral. Boehmite, although far less abundant than diapsore, may have paragenetically preceded diaspore in some deposits. Chlorite, presumably an Al-rich variety because the content of MgO is typically <0.5%, is also present. Li, which is sporadically present, is inferred to have accumulated in the chlorite which may be a proto-variety of cookeite. Because Li+ and Al3+ are similar in size, Li is inferred to have accompanied Al as a resistate element in contrast to K and Na which were leached from parent phyllosilicates.

It is a high honor to be invited to give this first Brindley Memorial Lecture. I view it as taking the first step on a ladder, to be followed by suceeeding talks that climb higher to the pinnacle that George Brindley erected for us in clay mineralogy. If George were with us, he would be sitting on the front row, as usual, keeping the speakers and audience "honest" in our deliberations. In turn, I would be privileged to ask him personally to enlighten us with his valued opinion on the many questions I will be asking in this talk.

Manganiferous karst bauxites are rare on a worldwide scale. One such body, recently mined at Kincsesbánya, Hungary, has been studied by chemical, petrographic, X-ray powder diffraction, scanning electron microscopic, and energy dispersive X-ray analytical techniques. The bauxite deposits of Kinesesbánya are of Paleocene to Lower Eocene age; however, the enrichment of manganese in them was a much later, epigenetic process. Lithiophorite is the main Mn mineral in this bauxite and occurs chiefly in dusters of < 1-μm size crystallites. Well-developed crystallites, however, 5–10 μm in size, line the walls of many microfissures and voids.

The oxidation of pyritic bauxite and lignitic clays in the overlying beds apparently mobilized finely disseminated Mn and Fe. Downward-migrating acidic solutions were gradually neutralized, and Mn and Fe minerals precipitated. The manganiferous bauxite was found only along the eastern rim of heavily eroded Middle Eocene sedimentary rocks. Here, epigenetic oxidation and mobilization were optimum. Farther to the east, pyrite-rich overburden and bauxite were apparently eroded away before Fe and Mn could be mobilized.

Alteration products of andesite cobbles from wet soils formed in volcanic colluvial material were studied using petrographic, electron microscope, X-ray powder diffraction, and thermal techniques. Augite phenocrysts altered by congruent dissolution leaving voids which were subsequently filled with smectite. Plagioclase also altered to produce micrometer-size spheroidal aggregates of smectite. Halloysite was not observed within the altered cobbles, although it was abundant in the soil matrix. The formation of smectite in the altered cobbles was probably favored by the restrictive drainage of the microenvironment in combination with wet soil conditions.

Montmorillonite and K-feldspar deposits of potential economic interest occur in the Late Oligocene-Miocene tuffs of the Vizcarra Formation in the state of Durango, Mexico. The two minerals were formed separately from rhyodacitic to rhyolitic pyroclastic deposits in a closed hydrologie system and diagenetically altered following two different patterns. In material deposited on dry land the glass was completely replaced by K-feldspar, and the interstices between the replaced glass bubbles and shards were filled with chalcedony, quartz, and albite. Pyroclastic material deposited in an alkaline lacustrine environment were diagenetically altered to montmorillonite, which formed the bentonitic tuffs widely exposed beneath the K-feldspar-rich tuff. These bentonitic tuffs contain as much as 85% montmorillonite plus authigenic chalcedony and quartz. Pyrogenic sanidine, quartz, and oxybiotite, coarse glass shards, and clastic grains make up about 8% of the clay-rich tuffs. The composition of the montmorillonite corresponds to the formula (Si3.88Al0.12)(Al1.41Mg0.59)O10(OH)2(Ca0.07Mg0.11,Na0.28K0.06). The montmorillonite is dioctahedral, the surface acidity is of the Lewis type, and the clay swells to one- and two-layer complexes. The cation-exchange capacity is 64 meq/100g; base exchange is Ca2+, 15; Mg2+, 20; Na+, 4.1; and K+, 1.0 meq/100 g. Its interlamellar charge is 18.6 microcoulombs/cm2. Dacitic pyroclastics were deposited later at higher elevations along the margins of the basin. Percolating solutions apparently removed a siliceous leachate from the dacitic glass and partially altered it to clinoptilolite.

The formation of iddingsite by the oxidative weathering of Fo80 olivine begins by solution of Mg from planar fissures, 20 Å wide and spaced 200 Å apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks < 50 Å in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20–50 Å wide and 1–7 layers thick, form bridges 50–100 Å long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 × 100 × 200 Å (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

Resilication of bauxite produced kaolin at and beneath an old erosion surface on bauxite at the Alabama Street Mine of ALCOA in Saline County, Arkansas. The transitional alteration can be traced in morphology by scanning electron microscopy, (SEM) and in Al:Si ratio by energy dispersive analysis. In one illustrated example, the sequence of resilication took place within 1 mm thickness; in another, across 80 mm. The first morphologic alteration of gibbsite (bauxite) appears to be to allophane that occurs in micrometer-size plates which show elongate cracking and/or straight to highly curved elongate edges. The next phase is kaolinite, first in micrometer-size flakes followed by coarser flakes that grade into a zone of typical stacked kaolinite, likewise identified by X-ray powder diffraction. Notably large stacks and small flakes of kaolinite are intimately mixed in the SEMs, thus suggesting that unequal sizes of kaolinite crystals can grow during one episode of an in-situ genesis.

The influence of added sodium and calcium nitrate electrolyte on the particle aggregates in the colloid fraction of natural bentonite and kaolin was studied. Clays were flocculated in distilled water and various electrolyte concentrations. Aggregate size was studied by sedimentation analysis; the mean radius of the aggregates was plotted against the concentrations of Na+ and Ca2+. For bentonite, the mean radii decreased with an increase of Na+ and Ca2+ concentration, reaching a minimum; and further increases in concentration led to an increase of the mean radii of the aggregates. For kaolin, an increase in Na+ and Ca2+ concentration gave rise to an increase in the mean radii of aggregates.

Scanning electron micrographs showed different types of aggregates, depending on the physico-chemical conditions of a sedimentation process. In bentonite and kaolin sediments formed from a distilled water slurry, the dominant aggregate was an edge-face type. The small addition of salts to a bentonite slurry led to the formation of edge-edge-type aggregates; for kaolin edge-face-type aggregates formed, although within the microaggregate face-face associations were observed. The highest concentrations of electrolytes for sediments of both clays led to formation of compact, face-face-type aggregates.

A standardless method of energy dispersive X-ray fluorescence in conjunction with scanning electron microscopy was used to analyze selected areas of clay-size particles of talc, pyrophyllite, and kaolinite supported by a carbon planchet. Peak intensity ratios of fluorescing elements relative to silicon were converted directly to weight or mole ratios using conversion factors determined theoretically. The conversion factors depend upon particle thickness and mass adsorption coefficients of the sample for the elements analyzed. The effects of particle thickness become significant above ~0.1 μm. Without using particle thickness corrections, the mean molar ratios of metal to Si agreed to within 6.1,0.5, and 9.7% of the theoretical ratios for kaolinite, pyrophyllite, and talc, respectively.

Hornblende of the Carrol Knob mafic complex (southern Blue Ridge Mountains, North Carolina) has weathered under humid, temperate conditions. Hornblende weathering appears to have been a dissolution-reprecipitation reaction, in which hornblende dissolved stoichiometrically, and the ferruginous and aluminous weathering products (goethite, gibbsite, and kaolinite) precipitated from solution (neoformation). During the earliest stage of alteration, ferruginous weathering products formed as linings of fractures within and around crystals and cleavage fragments of hornblende. Side-by-side coalescence of lenticular etch pits during more advanced weathering produced characteristic “denticulated” terminations on hornblende remnants in dissolution cavities bounded by ferruginous boxworks. Dissolution cavities are devoid of weathering products. Small “pendants” of ferruginous material project from the boxwork into void spaces. Because these products are separated from the hornblende remnants by void space, they must have been produced by dissolution-reprecipitation reactions. Complete removal of the parent hornblende left a ferruginous microboxwork or “negative pseudomorph.” Only Al and Fe were conserved over microscopic distances; alkali and alkaline-earth elements were stoichiometrically removed from the weathering microenvironment during the weathering process.

The Rokle bentonite deposit is part of an accumulation of argillized volcanoclastic rocks in the Tertiary stratovolcanic complex of the Doupovské Mountains east of Karlovy Vary (Carlsbad), about 100 km westnorthwest of Prague, Czechoslovakia. The arenite basalt ash was originally composed of hyaloclasts and subordinate biotite. The following types of montmorillonite aggregates were produced during the alteration of the ash in a stagnant, lacustrine environment: (1) extremely fine-grained micro-crystalline aggregates that have honeycomb textures and that replace the original hyaloclasts, and (2) coarse crystalline aggregates that have more open honeycomb textures and that fill pores and cracks in altered hyaloclasts and in pumice vesicles. Both types of aggregates have the same chemical composition according to energy dispersive X-ray analysis.

Montmorillonite, separated as the <1-¼m size fraction from olive gray bentonite, has the following crystallochemical formula: (K0.09Na0.02Ca0.29Mg0.10) (Al2.43Fe3+1.05Fe2+0.005Mn2+0.005Mg0.50) (Si7.62Al0.38) O20(OH)4. Biotite was apparently stable during the alteration of the hyaloclasts. Anatase and possible accessory heulandite-clinoptilolite were also formed in small amounts, but were not observed by scanning electron microscopy. Goethite is the youngest oxidation product in some parts of the bentonite. Minute fragments of sodium-rich plagioclase, potassium feldspar, quartz, and muscovite are ubiquitous accessories of the original hyaloclasts. Together with kaolinite, they formed from the underlying fresh or kaolinized orthogneiss.